Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(beginReq);
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(endReq);

function beginReq(sender, args) {
    if (window.parent.mostrarCargando)
        window.parent.mostrarCargando();

    if ($find(ModalProgress))
        $find(ModalProgress).show();
}

function endReq(sender, args) {
    if (window.parent.NoMostrarCargando)
        window.parent.NoMostrarCargando();

    if ($find(ModalProgress))
        $find(ModalProgress).hide();
} 
